1. Field of the Invention
The present invention relates to an optical system and a lens for ultraviolet (UV) radiation and, more particularly, to an optical system and lens for projecting an ultraviolet ray onto a wafer to form an image of a fine pattern.
2. Description of Related Art
In an image projecting system which reduces and projects a fine pattern of an integrated circuit (IC) or a large-scale integrated circuit (LSI), etc, onto a semiconductor substrate, UV radiation such as that produced by an excimer laser has been widely used in response to a demand to shorten the wavelength of light to be used in order to increase the density of the integrated circuit.
Conventional optical glass can not be incorporated into a UV optical system since the transmittance of conventional optical glass for visible light is insufficient for UV radiation To this end, a lens made of quartz (SiO.sub.2) or fluorite (CaF.sub.2), which has a high UV transmittance, is usually used instead of a conventional glass lens. However, the refractive index of quartz or fluorite is lower than that of a glass lens for visible light and hence it is impossible to directly apply the same lens design for a visible region or a near UV region to a quartz or fluorite lens. To compensate aberrations, the lens for UV radiation tends to become large.
There are two major problems with a large lens. First, a focusing mechanism becomes large. It is necessary to precisely adjust the focus in order to obtain a resolution at the diffraction-limited. To adjust the focus, either the lens or the wafer is moved in the optical axis direction. A wafer moving mechanism (stepper) is provided to move the wafer in a direction perpendicular to the optical axis. If the lens becomes large, then it is necessary to provide a large and complicated moving mechanism for the lens or wafer.
Secondly, it is difficult to evaluate the optical performance of the lens. In a UV projecting optical system which is aimed to form an image with a resolution at the diffraction-limited, the amount of aberrations remaining in the optical system must be decreased in proportion to the wavelength. To this end, it is necessary to enhance the production and precision of the lens for UV radiation, and also the lens should be adjustable to improve precision. This is far more important for a lens for UV radiation, than a glass lens for visible light. In particular, if a highly precise adjustment can be carried out, the level of production and precision would be reduced to some extent, thus facilitating the production of UV lenses.
In a lens which is adapted to form an image of a fine pattern, for example, in optical lithography, that is, in a lens which projects a micro pattern of an IC or LSI having a reduced size, it is desirable to use an image forming optical system having a light source with short wavelength, since the size of the smallest spot image which can be resolved is in proportion to the wavelength.
It is necessary to minimize the Petzval sum as much as possible in order to restrict the curvature of field. The Petzval sum P is represented by P=.SIGMA..phi./n, wherein ".phi." designates the power of each lens surface, and "n" the refractive index of each lens, respectively. In view of this relation, in conventional lenses for visible light, the curvature of field is corrected by a combination of glasses having different refractive indexes. However, since the material of which a UV lens is made is practically limited to vitreous silica or quartz (SiO.sub.2), or fluorite (CaF.sub.2), the above-mentioned solution can not be applied to UV lenses. In general, the transmittance of optical glass dramatically decreases in the UV region, and this decrease occurs more remarkably as the refractive index increases.
At the short wavelength, below about 300 nm, the material of the lens which can be used is limited to only SiO.sub.2 or CaF.sub.2. However, since fluorite is soft and hard to shape, in practice, the optical material available for the lens is limited to only vitreous silica or quartz. This limitation makes it impossible to eliminate the aberrations by a combination of lens materials having different refractive indexes. Consequently, it is fundamentally difficult to restrict the Petzval sum or a spherical aberration